(1) Field of the Invention
The present invention relates to an impurity diffusing furnace, and particularly to an impurity diffusing furnace in which the gas pressure within an atmoscan tube of the diffusing furnace is maintained at a constant level.
(2) Description of the Prior Art
FIG. 1 schematically illustrates the conventional impurity diffusing furnace. As shown in this drawing, an atmoscan tube 1 with a plurality of wafers (not shown) loaded on an internal cell boat 40 is introduced into a processor tube 2, and an amount of process gas is injected through a gas inlet 20, maintaining a certain constant temperature on a heater block 30, thereby diffusing an impurity into the wafer. The atmoscan tube referred to is of the type produced by BTU International, and identified as "3400 H-3800 H Systems".
In such an impurity diffusing furnace, in order to constantly maintain the internal gas pressure of the atmoscan tube which affects the impurity diffusing state of the wafer, the gas is discharged to the outside through a discharge tube 70 which is provided on a door flange 60 of the atmoscan tube 1, the discharge being made in accordance with the internal gas pressure of the atmoscan tube 1. In the drawing, reference code 50 indicates a fixed baffle.
However, if the discharge of the gas doesn't keep constant, variations in the temperature and the diffusing speed occur, thereby making it impossible to obtain uniform wafers. For this reason, the conventional impurity diffusing furnace has to be operated in such a manner that the gas is introduced in the rate of 15-20 slm (standard liter per minute) which is an excessive amount compared with the reaction mechanism of the wafer and the internal volume of the atmoscan tube 1, with the result that the loading capability becomes low.
Further, depending on the difference of the level within the discharge tube (70), the vent pressure gage (not shown) of the gas outlet will show different gas discharge rate, and therefore, the uniformity of wafers will deteriorate, thereby causing difficulties in the function.